Bidmas again! 9-9+9

[redhat]

I can't believe I am failing to assist my child with year 5 homework!

9-9+9

Is this 18 because you do the addition before the subtraction? (I thought it was this but think I'm wrong)

Or is it 9 because the addition and subtraction rank equally and so you go left to right?

I knew I should have paid more attention in school......

[user789653241]

I think the key to order of operation is just remember
BODMAS/BIDMAS and what the letter stands for, or like my ds, do it until it's actually stuck in your head.( My ds played BIDMAS Blaster until he got gold shield for everything so now he doesn't even need to think about it.)

[Thecatisatwat]

Redhat I agree with you, I would have got -9 as well (and I got an A at o-level maths a year early). When did D and M and A and S become equal (obviously sometime between 1985 and today) and how do you decide when one takes precedent? Thought I'd be fine helping dd with maths at primary but clearly not.

[HerbWoman]

The sign is attached to the number it is in front of. If you find it hard to see what's going on here, try visualising it on a number line - start at 0, then move right by 9 (+), then left by 9 (-), then right by 9 again (+). The only way you would get 18 to take away would be if the second two 9s were in brackets with the minus sign outside, as in 9-(9+9), which really means 9-1(9+9). When multiplied out, this gives 9-9-9, which gives you the answer -9. Sorry if that just confuses you more (it makes sense in my head).

[ouryve]

• and - are the same priority, so just read it as a sentence from left to right. Better to look at it as
  
  B I DM AS

[ouryve]

And + and - have always been equal, since subtraction is simply adding a negative number.

In the same way, division is multiplying by a fraction.

[mumsneedwine]

Addition first. So 9+9=18.
Then subtraction so 9-18=-9

It's bracket, indices, division, multiply, add, subtract. In that order

[mumsneedwine]

You cannot change numbers around. So it's not 18-9.

[redhat]

I'm happy that its 9 now but glad to see I'm not the only one who would have thought it was -9

That's the problem with just a little bit of knowledge sticking!!

[mumsneedwine]

Redhat it is -9. Each operation has to be done in a specific order. There is no equal weighting !!

[redhat]

No it isn't. Honestly. I thought so too but addition and subtraction have equal ranking and so you go left to right (or so I'm told by everyone else on the thread!) This also accords with the answers to DS's homework!

[PurpleDaisies]

Redhat it is -9. Each operation has to be done in a specific order. There is no equal weighting !!

mums you're wrong because you've forgotten the minus sign belongs to the number after it.

You could rewrite the sum as
9 + -9 + 9. If you add the second numbers you get 0, so the final answer is 9.

[PurpleDaisies]

When did D and M and A and S become equal (obviously sometime between 1985 and today) and how do you decide when one takes precedent?

I have no idea when they started teaching this in school )I did it got my maths GCSE in 1998 but it doesn't matter which one you do first-neither takes precedent. You can go left to right or do whatever makes the sun easiest
Eg 10 X 6 / 2
You can do 10 X 6 = 60 then 60/2 = 30
Or
6/2 = 3 then 3 X 10 = 30.

[dementedpixie]

Multiplcation/division done first (but they are both of equal importance). Once that is done you can do addition/subtraction working your way in turn from left to right (they are of equal importance to each other too)

[mumsneedwine]

They don't have equal weighting. You must do subtraction last. That's the whole point of the task.

When it comes to more complex sums it's vital to do in right order.

I agree this is -9+9=0 and then 9+0=9.
Teach me to not read the question correctly !!!!

So for 4x12/6x5+5-3

It'd divide first, so 12/6=2

Then it's 4x3x5+5-3

Multiply next so 5x3x5 =75

Then add so 75 +5=80

Then subtract 80 -3 = 77

[dementedpixie]

You are wrong. Did you not read the links

[PurpleDaisies]

They don't have equal weighting. You must do subtraction last. That's the whole point of the task.

I'm sorry redhat but you're not right. I teach GCSE and A level maths. Some schools teach a strict adherence to BODMAS so students don't get confused but mathematically addition and subtraction can be done in either order. So can multiplication and division.

[dementedpixie]

In your example, in the absence of brackets, you do the multiplication/division in the order they appear before doing the addition and subtraction at the end

[redhat]

I don't think you're right mums. You say you must do subtraction last but then when you work the question you've done it first.

Anyway I was wrong

I did 9-9+9
9+9 (because you do addition first)
then 9- 18 (because 9+9 was 18)
= -9

I now know I just go from left to right which gives me 9 and which is the logical way I would have done it before DS's homework made me remember bidmas!

DS had some more of this to do last night and there was a worked example on the computer programme which actually talked you through the question and some examples and do say "if you have an addition and a subtraction you take them from left to right"

This is all quite embarrassing since I was really good at maths at school!

[redhat]

I wasn't me purple - I'm convinced!

[dementedpixie]

Mumsneedwine I get the answer to be 42 btw

[Librariesgaveusp0wer]

Addition and subtraction cannot come at different points in the order of operations because they are, at a fundamental level, the same thing. Subtracting 9 is the same as adding -9. You couldn't have those things having different results on a sum.

[PurpleDaisies]

Taking your example, multiplying first:
4x12x5=240
Then divide...
240/6 =40
Then subtract
40-2=38
Then add
38+5=43.

You've made mistakes but doing it your way correctly also gives 43.

[PurpleDaisies]

Oops, the answer is 42 not 43!

[PurpleDaisies]

Sorry, it really is 43!

[redhat]

42?...